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Chapter 11

BIOLOGY 1M03 Chapter 11: Chapter 11- From Hominoid to Hominin

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Ben Evans

Chapter 11: From Hominoid to Hominin - during Miocene, Earth’s temperature began to fall because: o total amount of rain declined each year o rainfall became more seasonal, so there were several months each year when no rain fell - some animals failed to adapt and became extinct - spread of woodland and savanna led to evolution of first hominins about 6 mya - hominins were different from any of the Miocene apes in two ways: 1. they walk upright ▪ bipedal locomotion led to major morphological changes in bodies 2. in new savanna and woodland habitats, new kinds of food became available ▪ caused changes in teeth, jaws, and skulls - 5 categories distinguish modern humans from contemporary apes: 1. We habitually walk bipedally. 2. Our dentition and jaw musculature are different from those of apes in a number of ways. 3. We have much larger brains in relation to our body size. 4. We develop slowly, with long juvenile period. 5. We depend on an elaborate, highly variable material and symbolic culture, transmitted in part through spoken language. - hominins are not included in the same genus (Homo) as modern humans At the Beginning - last common ancestor of humans and chimpanzees lived between 7 and 5 mya - Ardipithecus ramidus, Orrorin tugenensis, and Sahelanthropus tchadensis have begun to shed light in history of human lineage Ardipithecus - genus Ardipithecus include two species (A. ramidus and A. kadabba) and both have similarities to both humans and chimpanzees - several features suggest that A. ramidus was a hominin o opening on bottom of skull through which spinal cord passes (foramen magnum) is located forward under the skull, as it is in humans o forward placement of foramen magnum is associated with bipedal locomotion o were smaller, more incisorlike canine teeth that are not sharpened by the lower premolar, unlike apes, which have relatively large canines o enamel is thinner than in other early hominins, and the canines are smaller than in chimpanzees and gorillas but larger than in later hominins o seemed to have lived in a forested environment ▪ based on fossils of wood and seeds found at site - unclear whether Ardipithecus is related more closely to humans or to chimpanzees 1 Orrorin tugenensis - second early fossil with similarities to humans - incisors, canines, and one of the premolars are more like with the teeth of chimpanzees than of later hominins - arm and finger bones have features that are believed to be adaptations for climbing - thigh bones are more similar to those of later hominins than to those of apes - habitat was a mix of woodland and savanna Sahelanthropus tchadensis - fossil consists of a nearly complete cranium, two fragments of lower jaw, and several teeth - fossil shook paleontological community because: 1. it came from an unexpected place ▪ most work on human evolution has focused on East Africa and South Africa, but this specimen comes from the middle of Africa • suggests that hominins had a larger range than believed 2. this fossil is surprisingly old ▪ geology of site does not allow radiometric or paleomagnetic dating ▪ oldest hominin cranium 3. possesses a very surprising mix of anatomical features ▪ face is relatively flat and massive browridge over eyes The Hominin Community Diversifies - a number of hominin species lived in Africa between 4 to 2 mya o they are divided into three genera: Australopithecus, Paranthropus, and Kenyanthropus 1. Australopithecus o includes 6 species: A. anamensis, A. afarensis, A. africanus, A. garhi, A. habilis, and A. rudolfensis o the australopithecines had small bipeds with teeth, skull, and jaws adapted to a generalized diet 2. Paranthropus o includes 3 species: P. aethiopicus, P. robustus, and P. boisei o similar to the australopithecines from neck down, but had massive teeth and jaws adapted to heavy chewing of tough plant materials, and a skull modified to carry enormous muscles necessary to power chewing apparatus 3. Kenyanthropus o includes 1 species: K. platyops o distinguished by a flattened face and small teeth Australopithecus - A. anamensis o bipedal, but had a more apelike skull than later australopithecines had o found near Kenya o found parts of upper and lower jaw (part of tibia- larger of two bones in lower leg) and numerous teeth 2 o large molars with thick enamel and smaller canines, and the shapes of knee and ankle joints strongly indicate it was bipedal o arm bones suggest that it retained adaptations for life in trees o ear holes are small and shaped like ellipses, as they are in living apes, but ear holes of later australopithecines are larger and more rounded o dental arcade is more like U shape (seen in chimpanzees and gorillas), unlike the V-shaped dental arcade seen in later australopithecines o lived in a mixture of habitats including dry woodlands and gallery forest along rivers - A. afarensis o most extensive fossil collections come from several sites in Ethiopia o found bones of a 3 million year old knee joint that showed striking similarities to a modern human knee o also discovered fossils in Tanzania o habitats ranging from woodland to dry savanna o cranium (skull minus lower jawbone) is quite apelike o its endocranial volume (capacity of brain cavity) is small ▪ about size of modern chimpanzee o front of case below the nose is pushed out- a condition known as subnasal prognathism o jaw point is shallow o teeth and jaws are intermediate between those of apes and humans 1. dental arcade of A. afarensishas an intermediate V shape, the canines are medium-sized, and the diastema (space between teeth and jaw) is modest 2. in its canines, A. afarensis displays less sexual dimorphism than chimpanzees but more than modern humans 3. first lower premolar has a small inner cusp and a larger outer cusp • chimpanzees have a single cusp, whereas humans have two cusps of approximately equal size o was bipedal Cranium of A. afarensis 3 Teeth and Jaws of Australopithecus afarensis Pelvis of A. afarensis 4 Schematic Diagram of Lower Body at Point of Stride o although they are bipedal, it is not clear whether this hominin had same striding pattern as modern humans because many features in the pelvis and legs are different than those from modern humans o have shorter legs than modern humans o pelvis is much wider, which means minimized vertical motion of body during walking o probably spent a good deal of time in trees o traits of feet, hands, wrists, and shoulder joints suggest that they were partly arboreal o variation in size of A. afarensis adults ▪ could represent sexual dimorphism ▪ could also represent two different species • bigger one could have adapted to savanna and smaller one could have adapted between tree climbing and walking - A. africanus o base of skull has fewer air pockets, face is shorter and less protruding below the nose, the canines are less dimorphic, and the base of cranium is bent further upward, or flexed o molars are quite big, and lower jaw is larger and sturdier than in modern humans o matured rapidly, unlike humans o if we know rate at which teeth developed in extinct hominins, we can estimate how rapidly they developed overall ▪ enamel is secreted as teeth grow o developed faster than chimpanzees, not slowly like humans ▪ these results show that australopithecine infants did not have as long a period of dependency as human children do 5 - A. garhi o lived about 2.5 mya in East Africa o discovered a number of postcranial bones and partial skeleton of one individual in Ethiopia o later discovered pieces of skull, maxilla (upper jaw), and teeth come from the same stratigraphic level as postcranial remains o had small brains, like A. afarensis and A. africanus o canines, premolars, and molars were generally large o also had a sagittal crest, a fin of bone that runs along the centerline of the skull (like a Mohawk haircut) o do not reveal some interesting developments in hominin lineage o resembles chimpanzees in proportions of bones (humerus, the radius and
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